Over the past several years there has been, and continues to be, a tremendous amount of activity in the area of efficient encoding of speech. A large number of digital coding algorithms are being investigated for a wide variety of applications. For an evolving digital telephone network, a most important application is the possible replacement of 64,000 bit-per-second (b/s) PCM signal (8 bits per time slot, repeated at an 8 kHz rate) with other coding algorithms for telephone--both in the public switched and private line networks. The reason, of course, is to achieve bandwidth compression. The dilemma for telephony planners is easily posed but not so easily answered--should such a network evolve toward a coding algorithm more efficient than 64 kb/s PCM and, if so, which algorithm is preferable? A number of different digital coding algorithms and related techniques have been proposed heretofore, namely: Adaptive Differential PCM (ADPCM); Sub-Band Coding (SBC); Time Domain Harmonic Scaling (TDHS); vocoder-driven Adaptive Transform Coding (ATC), etc.
For a realistic mix of input speech and voiceband data, the ADPCM approach appears to be the most promising. One form of adaptive differential PCM coding is disclosed, for example, in my copending application Ser. No. 343,355, filed Jan. 27, 1982 now U.S. Pat. No. 4,437,087 and can be considered a benchmark since a single encoding with this coder at 32 kb/s is near to being subjectively equivalent to 64 kb/s .mu. 255 PCM. In some networks it is possible to have multiple conversions from 64 kb/s PCM-to-32 kb/s ADPCM-to-64 kb/s PCM. In these applications, it is important to eliminate accumulation of distortion in the multiple PCM-to-ADPCM-to-PCM code conversions.
An attempt towards this goal is described in an article by Hideyo Murakami entitled, "A Low Noise ADPCM-LOG PCM Code Converter", 1979 International Symposium on Circuits and Systems Proceedings, IEEE catalog No. 79 CH1421-7 CAS, pages 969-970. However, this arrangement assumes a uniform ADPCM quantizer and distortion could result in those applications in which a non-uniform quantizer is employed.